Microplastics (MPs) are emerging persistent contaminants in the terrestrial subsurface, and evidence has emerged for significant effects of MPs on the biological and ecosystem functions of soils. Main MP sources include land spreading of sewage sludge and biowaste composts, plastic mulching film used in horticultural fields, waste water irrigation and leachate from the landfills, among others. This updated state-of-the-art review paper describes recent experimental and numerical research and developments in understanding the accumulation and fate and effects of MPs in the soil environment (focusing on their storage, degradation, transportation, leaching to groundwater etc.), followed by mitigation and bioremediation measures, including MP-eating soil bacteria and fungi and the best management practices for reducing MP pollution of soil. Other areas covered are the combined effects of MPs and various other environmental contaminants (heavy metals, organic pollutants and antibiotics) in soil ecosystems and the standardisation of methods for detection, quantification and characterisation of MPs in soils, which is critical for MP research. The paper concludes by identifying knowledge gaps and presents recommendations on prioritised research needs.
Biosolids/sewage sludge materials are the residue by-products of municipal wastewater treatment. Drainage of biosolids storage facilities is a critical aspect of their design. Laboratory permeability tests are widely performed for economical and practical reasons. Reported laboratory coefficient of permeability (k) values for these materials suggests very/extremely low permeability. However, limited field data reported for sewage sludge material would tend to suggest significantly greater k values. This briefing article explores issues related to the water permeability (filter size) of the porous media (discs) in contact with the specimen drainage boundaries during laboratory consolidation and permeability testing of biosolids/sewage sludge materials. These include clogging of their pores by fine soil particles and biogenic gas bubbles, bio-clogging, and the increase in viscosity of the pore liquid remaining within the test specimen due to filtering action. In applications where rates of flow (and consolidation) are measured, their effect can be to produce misleading results, potentially by many orders of magnitude. Recommendations on laboratory consolidation and permeability testing, including selection and verification of appropriate porous media, are presented. Issues relating to the analysis and interpretation of experimental consolidation data are discussed, with caution urged on the use of classical consolidation theory for these difficult and unconventional geomaterials.
This state-of-the-art paper discusses determinations of remoulded undrained shear strength (s u) for municipal sludge and residue materials by the fall-cone approach, used increasingly for this purpose in practice. The strength mobilised by a falling cone as it penetrates into a soil test-specimen is related to its ‘static’ strength s u (used in design for most conventional loading cases) through a rate dependence parameter. From a review of the literature and analyses of existing and new experimental data, compared with inorganic soils, these highly organic soils are found to exhibit significantly greater strain-rate dependence for strength, which reduces correspondingly the value of Hansbo’s cone factor K. Hence, in the absence of reported experimental K values for different organic soils, use of inorganic soil K values in strength calculations can often result in significant overestimations of undrained strength. Recommendations are made regarding the cone characteristics more suitable for such testing and experimental approaches useful for determining related K values. Other influencing factors, including the fibrous nature of some organic soils and cone roughness (adhesion), are discussed. The paper concludes by presenting semi-logarithmic and power model formulations for s u determinations from measured water contents as interim (alternative) methods until the mentioned issues are satisfactorily resolved.
In order to prevent possible charring, oxidation and (or) vaporisation of substances other than pore water, many researchers have adopted oven-drying temperature (t) values in the range 60–90°C for water content determinations of peat and other highly organic soils. This paper investigates the oven-drying characteristics of six very different highly decomposed peaty soils retrieved from south-west China for t values of 65, 85 and 105°C. Based on the presented experimental data, it is concluded that the standardised t value of 105°C used for testing of inorganic soil is also appropriate (and preferable to using lower t values) for routine water content determinations of these soils, which is in line with the growing consensus formed by other researchers on this topic. Further, the authors recommend a wet specimen mass of approximately 50 g and a 24 h oven-drying period in performing the oven-drying tests for t = 105°C. The paper also describes an existing approach to comparing and standardising water content values determined for the same organic soil, but on the basis of different t values, with the new data presented herein for the six Chinese soils used to strengthen its wider applicability in geotechnical engineering practice and related fields.
Reverse extrusion has been promoted over the past four decades for remoulded undrained shear strength (s u) measurement and consistency-limit determinations. The technique employs a uniaxial-compression test machine to set the die travelling against the confined soil specimen (billet) at a constant displacement rate (v), with the soil extruded through the die orifice under a steady-state extrusion pressure (p e). This paper presents the first comprehensive literature review of this topic, including a reassessment of extensive data sets presented for many hundreds of fine-grained soils covering an extremely wide plasticity range. Specifically, the paper critically examines (a) the soil billet’s assumed undrained condition and hence the constancy of the p e/s u ratio value, which is central to s u determinations using this approach; (b) the dependence of the steady-state p e value on both the billet’s area reduction ratio (R) and v; (c) the role of soil remoulding toughness; and (d) the pitfalls of various data analysis and interpolation techniques employed. The author concludes that, depending on mineralogy and gradation, localised billet consolidation may occur for the slow displacement rates employed and high p e values required, particularly for stiffer soils, such that the present extrusion approach is generally not recommended for s u measurement or Atterberg-limit determinations.
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